Bladder Purinergic Receptorsa

Abstract

In rabbits the contractile response of the urinary bladder is only partially due to cholinergic innervation since atropine does not completely block neuron ally mediated contractions. In the human bladder this atropine resistance is controversial with some reporting atropine resistance in­ vitro while others have stated that the atropine resistance is also tetrodotoxin resistant. Results of the present investigation demonstrate that an atropine resistant, tetrodotoxin sensitive contraction can be evoked in some, but not all human bladder strips. Evidence accumulated over the past few decades indicates that this atropine resistant contraction may be mediated by ATP or a related purine compound. Studies presented herein are designed to develop a radioligand assay for this receptor. Initial studies indicated that the hydrolysis resistant ATP analog (3,1' methylene ATP offers several advantages over ATP as a potential radio ligand. It is only slowly hydrolyzed by endogenous ATPase and does not inhibit the hydrolysis of ATP indicating that it probably does not bind to the active sites of endogenous ATP hydrolyzing enzymes. In addition (3,1' methylene ATP is 10-100 fold more potent than ATP itself in stimulating contractions of the urinary bladder in-vitro. The radio ligand binding assay herein described can be used to quantitate the density of receptors, an essential step for determining the role of this system in urinary bladder function and dysfunction. Application of this assay could form the foundation for development of a new class of therapeutic agents for the treatment of urinary bladder dysfunction based on modulation ofthe nervous system. (J. Ural., 144: 176-181, 1990) The postganglionic nerve evoked contractile response of the urinary bladder of nearly all vertebrate species including man is only partially blocked by antimuscarinic agents such as atropine and scopolamine ' and is relatively unaffected by alpha and beta adrenergic or nicotinic antagonists.! The most likely explanation for this response is that in addition to acetylcho­ line, released by cholinergic nerves, another neurotransmitter released by non-adrenergic, non-cholinergic nerves contributes to the contraction. The presence of these neurons has been alluded to in the early literature by Langley2 and their existence has been suggested throughout the gastrointestinal tract as well as other organs including lung, trachea, esophagus, cardiovas­ cular system, and seminal vesicles.! In the human female blad­ der these nerves have been estimated to be responsible for 50% of the contractile response. 3 Evidence accumulated over the past few decades suggests that the neurotransmitter utilized by these neurons is most likely a purine nucleotide or analog and these nerves have therefore been coined purinergic by Burns­ tock Functionally these neurons are those that produce an effect on the innevated tissue in the presence of both atropine and guanethidine. Although other explanations for the atropine resistant re­ sponse have been put forth including the presence of a barrier to the penetration of atropine, there is little evidence for them. Many substances other than ATP have been explored as can­ didates for mediating the atropine resistant contraction. These include catecholamines, 5-hydroxytryptamine, cyclic AMP, his­ tamine, prostaglandins, alanine, arginine, glycine, glutamic acid, gamma amino butyric acid, bradykinin and substance P.! These substances have been rejected as candidates by most workers since they a) were either inactive or did not mimic the nerve mediated response, b) specific antagonists for these sub